Temperature compensated fluid level sensor

ABSTRACT

An apparatus for detecting the presence or absence of fluid at a predetermined level within a container. The apparatus comprises an electrical circuit and a sensor device electrically joined to the circuit and positioned within a wall of the container. The device comprises a housing of conductive material, a conducting member within the housing and insulated therefrom, a tip member of conductive material affixed to the housing, a resistive element having first and second opposing ends, the first end is electrically connected to the conducting member and the second end electrically connected to the tip member, and first and second oppositely aligned similar bimetallic members affixed respectively to the first and second opposing ends of the resistive element and adapted for engaging only when the temperature difference between the first and second opposing ends of the element exceeds a predetermined level.

United States Patent [1 1 Szeverenyi Nov. 13, 1973 TEMPERATURECOMPENSATED FLUID LEVEL SENSOR [75] Inventor: Nikolaus A. Szeverenyi,Warren, Pa.

[73] Assignee: GTE Sylvania Incorporated, Seneca Falls, N.Y.

[22] Filed: Mar. 20, 1972 [21] Appl. No.: 236,148

[52] US. Cl. 340/244 R, 337/380 [51] Int. Cl. G08b 21/00 [58] Field ofSearch 340/244 R; 337/380 [56] References Cited UNITED STATES PATENTS2,304,211 12/1942 Sparrow 340/244 R 3,171,934 3/1965 Brennan 337/380Primary Examiner-Thomas B. Habecker Attorney-Norman J. OMalley et a1.

[5 7] ABSTRACT An apparatus for detecting the presence or absence offluid at a predetermined level within a container. The apparatuscomprises an electrical circuit and a sensor device electrically joinedto the circuit and positioned within a wall of the container. The devicecomprises a housing of conductive material, a conducting member withinthe housing and insulated therefrom, a tip member of conductive materialaffixed to the housing, a resistive element having first and secondopposing ends, the first end is electrically connected to the conductingmember and the second end electrically connected to the tip member, andfirst and second oppositely aligned similar bimetallic members affixedrespectively to the first and second opposing ends of the resistiveelement and adapted for engaging only when the temperature differencebetween the first and second opposing ends of the element exceeds apredetermined level.

19 Claims, 5 Drawing Figures PATENTED NOV 13 I973 En l TEMPERATURECOMPENSATED FLUID LEVEL SENSOR BACKGROUND OF THE INVENTION Thisinvention relates to fluid sensing apparatus and more particularly to anapparatus for providing a signal indication when the fluid in acontainer is below a predetermined level.

. Previous known methods for detecting the level of fluid whthin acontainer have varied from mechanically operated floats to probingdevices requiring elaborate electronic circuitry. A particularshortcoming to many devices of the former variety has been the inabilityto compensate for a change in temperature of the fluid being measured.Additionally, those devices able to compensate for fluid temperaturechanges have, as mentioned, required extensive electronic circuitrywhich in turn has added appreciably to the complexity of operation ofthe device as well as to the costs for manufacturing such items.

It is believed, therefore, that a device for detecting the presence orabsence of fluid at a predetermined level within a container which wouldcompensate for varying temperatures of the fluid being measured as wellas be relatively simple in operation and inexpensive to manufacturewould constitute an advancement in the art.

OBJECTS AND SUMMARY OF THE INVENTION It is therefore a primary object ofthis invention to provide a fluid level sensing apparatus which includesa means for compensating for possible varying temperatures of the fluidto be detected.

It is a further object of this invention to provide a sensing apparatuswhich operates in a relatively simple manner and is relativelyinexpensive to manufacture.

In accordance with one aspect of this invention there is provided anapparatus for detecting the presence or absence of fluid at apredetermined level within a container. This apparatus comprises anelectrical circuit having a potential source, a switching means foropening and closing the circuit, and a current indicating means forindicating when the current in the circuit exceeds an established level.Additionally, the apparatus comprises a sensor device electricallyconnected to the circuit and having a housing, an electricallyconducting member positioned within the housing and insulated therefrom,a tip member bonded to the housing, a resistive element having first andsecond opposing ends of conductive material, the first end iselectrically connected to the conducting member and the second endelectrically connected to the tip member, and first and secondoppositely aligned bimetallic members possessing a substantially similardegree of thermal deflection and affixed respectively to the first andsecond opposing end of the resistive element. These bimetallic membersare adapted for engaging when the temperature difference between theopposing ends of the resistive element and hence the temperaturedifference between the bimetallic members exceeds a predetermined level.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a side elevational view ofone embodiment of the present invention.

FIG. 2 is an enlarged view of the sensor device of FIG. I.

FIGS. 3 and 4 are enlarged views of various phases of operation of thesensor of FIG. 2.

FIG. 5 is a plan view of the contacting ends of the expandable membersof the sensor device as taken along the line 5-5 in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS For a betterunderstanding of the present invention, together with other and furtherobjects, advantages and capabilities thereof, reference is made to thefollowing disclosure and appended claims in connection with the abovedescription of some of the aspects of the invention.

In FIG. 1 one embodiment of a liquid level detecting apparatus 10 inaccordance with the invention is illustrated and shown to comprise anelectrical circuit 11 and a sensor device 13. Circuit 11 comprises apotential source illustrated as battery 15, a switching means 17 foropening and closing circuit 11, and a current indicating means,illustrated as bulb l9. Sensor device 13 is electronically connected tocircuit 11 and is shown to be positioned within wall 21 of a fluidholding container 23. Although the particular method illustrated forpositioning sensor 13 is to place it in the side of container 23, sensor13 may be placed in either the bottom or top of the container dependingon the level of fluid desired or the configuration of the container.Furthermore, although the method shown for retaining sensor 13 in wall21 is by screw threads (the sensor being provided with external screwthreads to mate with corresponding threads in wall 21), other methodsfor retention are possible, an example being either welding orsoldering. The method illustrated is preferred, however, because itprovides relative ease of removal of sensor 13 in the event of damage tothe sensor or container.

In FIG. 2 can be seen a more detailed view of sensor 13, which is shownto comprise a housing 25 of electrically conductive material, anelectrical conducting member, illustrated as center post 27, which ispositioned within housing 25 and insulated therefrom by insulatingmaterial 29, a resistive element 31, and a tip member 33 of electricallyconductive material. Center post 27 is electrically connected to circuit11 via plug 35 illustrated as a body of insulative material 37 about ametallic socket 39 which in turn is connected to conducting wire 41.Plug 35 is but one means possible for providing this interconnection andis not meant as the sole method to whichthe invention is limited.Additional connecting means, including an alligator clip or even asingle wire soldered to post 27 are possible. In the event that theatmosphere surrounding container 23 is not favorable for exposedelectrical connections, a plug having an insulating cap to encompass theexternal portions of either post 27 or housing 25 is preferred.

Tip member 33, sealed in housing 25 to thereby encapsulate resistiveelement 31 therein, is adapted for being subjected to the fluid withincontainer 23. A preferred material for tip member 33 is sold under thetrade name Rodar and manufactured by the W. B. Driver Company of Newark,N.J., a subsidiary of the assignee of the proposed present invention.Rodar, consisting essentially of about 29 percent by weight nickel, 17percent by weight cobalt, and the remainder iron, is a suitableelectrical conductor and possesses the additional property of relativelylow thermal conductivity. This additional property, as will be furtherexplained, is highly desirable to enhance thefunctioning characteristicsof sensor 13. Resistive element 31, comprising first and second opposingends 43 and 45 .re-

spectively, is joined at first opposing end 43 to center post 27 and atsecond opposing end 45 to tip member 33. First and second opposing ends43 and 45 are of electrically conductive material and have first andsecond bimetallic members 47 and 49 affixed respectively thereto. Theconductive material preferred for opposing ends 43 and 45 is anickel-silver metallic alloy, although any of the well known metals ormetal alloys having good electrical conducting properties can beutilized.

Bimetallic members 47 and 49, also possessing similar electricalconducting properties, are illustrated as being oppositely aligned andin a non-engaging relationship when the fluid level in container 23 isabove tip member 33. As will be explained, these bimetallic members areadapted for engaging when the fluid level drops below the tip member.Electrical connection between circuit 11 and housing 25 is accomplishedsimply by affixing a wire from circuit 11 to any external portion of thehousing, using any of the conventional methods, for example, welding. Inthe event that container 23 is of metallic nature or any material havinggood electrical conducting properties, circuit 1 1 may be connectedthereto at any suitable location.

To explain the operation of apparatus 10, as shown in FIG. 1, particularreference is also made to items illustrated in detail in FIGS. 3 and 4.In FIG. 3, tip member 33 is shown as being subjected to the fluid withincontainer 23. As previously explained, bimetallic members 47 and 49 arein a nonengaging relationship when these members are at the sametemperature. These members are comprised of a suitable bimetal havingsimilar electrical conductive properties as well as similar degrees ofthermal deflection thereby permitting each member to deflectsubstantially equal, as the environmental temperature alters. A uniquefeature of sensor device 13 is its ability to operate in fluids having awide variety of temperatures, due to the positioning of bimetallicmembers 47 and 49. When the fluid surrounding tip member 33 isexcessively warm, expandable members 47 and 49 deflect equally to theposition illustrated in FIG. 3. In the event that the fluid is cooled,these members deflect accordingly in the opposite direction (shown inphantom). It is remembered, however, that members 47 and 49 maintain anon engaging relationship throughout these varying stages of deflection,provided tip member 33 is subjected to the fluid within the containerthereby keeping the temperature of all elements within the sensorsubstantially equal.

To operate apparatus 10, switching means 17 is closed, thereby providingelectrical current to circuit 11 and to sensor 13. A typical directionof current flow from battery is through center post 27, first opposingend 43 of resistive element 31, resistive material 51 of element 31,shown in FIG. 3, second opposing end 45 of element 31, tip member 33,housing 25 and thereafter back to circuit 11 where it passes throughbulb 19. Because this current must pass through resistive material 51,which may be any material typically found in electrical resistors, it isnot sufficient to activate bulb l9. Assuring that bulb 19 will not lightunder these conditions is easily accomplished by proper selection ofcorresponding elements in the sensor and circuit. One example of aworkable circuit-sensor arrangement is to use a 12 volt batteryconnected to a bulb having a resistance of approximately 1 ohm. Whenusing this combination, the desired resistance of the resistivematerialof element 31 is approximately 240 ohms. The resistance of otherelements in sensor 13, particularly housing 25, opposing ends 43 and 45,center post 27, tip member 33, and bimetallic members 47 and 49 isminimal and can be considered effectively as zero.

The current through resistive material 51 causes this material to becomewarm as is the case in almost all electrical resistors. This heat thendissipates out through opposing ends 43 and 45 of element 31. The heatdissipated through end 43 is heat sinked further through center post 27,insulative material 29, housing 25, and eventually into container wall21. The heat dissipated through end 45 is heat sinked primarily throughtip member 33 and then into the fluid within container 23. Provided tipmember 33 remains subjected to the fluid, the heat created in element 31is dissipated at a substantially equal rate through the above-describedchannels thereby maintaining the temperature of all members withinsensor 13 approximately the same. However, when the fluid level dropsbelow tip member 33, as illustrated in FIG. 4, an imbalance to this rateof dissipation is created. This unequal rate occurs primarily becausethe fluid, which previously served as a heat sink for the heat generatedin end 45 is now absent. However, to further assure this imbalanceduring a low fluid level, it is preferred that the overall volume ofcenter post 27 be substantially greater than the corresponding volume oftip member 33. It is also additionally preferred that the coefficient ofthermal conductivity of center post 27 be larger than that of tip member33, but this is not necessarily required provided a substantialdifference of volumes between these two members exists. In theparticular embodiment, the tip member comprised of Rodar has acoefficient of thermal conductivity of approximately 12.0 BTU/(hr.) (sq.ft.) (F per ft.) while that of center post 27 which is preferably ofsteel or similar composition, ranges between 25 and 40 BTU/(hr.) (sq.ft.) (F per ft.).

As described, the absence of fluid now causes end 45 to becomesubstantially warmer than end 43. This temperature difference in turncauses bimetallic member 49 to become warmer than bimetallic member 47which results in an unequal amount of deflection by metallic member 49to therefore upwardly deflect and engage member 47. When these twomembers engage (as shown in FIG. 4) electrical current from center post27 through first opposing end 43 is permitted to by-pass resistivematerial 51 and pass directly to second opposing end 45 via the engagedbimetallic members 47 and 49 because the combined resistance of members47 and 49 is effectively zero, as previously described. The current thenreturns to bulb 19 through tip member 33 and housing 25 where it now isat sufficient level to actuate the bulb. This in turn indicated to anoperator that the fluid level of container 23 is below tip member 33.

The sensor device as illustrated is unique in another feature in that itprovides a means whereby bulb 19 is sequentially actuated to morereadily indicate to the operator the low fluid level. This sequentialactuation is achieved when the electrical current by-passing resistivematerial 51 passes through engaged bimetallic members 47 and 49 for asufficient period to allow material 51 to cool. This cooling lowers thetemperature difference between opposing ends 43 and 45 to thereby reducethe corresponding temperatures in bimetallic members 47 and 49, causingthem to open. The current thereafter must pass through resistivematerial 51 once again until this temperature difference is againattained, causing members 47 and 49 to once again engage. This cycle oftemperature changes indirectly raises and lowers the level of currentflow in sensor 13 and circuit 11 and therefore causes bulb 19 to beenergized intermittently. If desired, however, a steady signal can beprovided by use of an electrical or mechanical latching circuit ordevice which latches on to energize bulb l9 continuously after the firstdeflection of bimetallic member 47. Such latching circuits or devicescan include an SCR, latching relay, or similar circuits and devices.

To adequately achieve the degree of deflection required in the operationof sensor 13, similar bimetals must be used for each of members 47 and49. More specifically, a bimetal utilized successfully in sensor 13 isChace 2,400 bimetal, manufactured by the W.M. Chace Company of Detroit,Michigan, a subsidiary of the previously mentioned W. B. Driver Company.Chace 2,400 bimetal has a high expanding side (illustrated as highexpanding layer 53 in FIG. 4) consisting essentially of about 22 percentby weight nickel, 3 percent by weight chromium, with the remainder iron,and a low expanding side (illustrated as low expanding layer 55 in FIG.4) consisting essentially of 36 to 42 percent nickel with the remainderiron. An additional newly available bimetal also found suitable for usein sensor 13 is one produced by the W.M. Chace Company having a highexpanding side consisting essentially of 22 percent nickel, 3 percentchromium, with the balance iron, and a low expanding side consistingessentially 30 to 35 percent nickel with the remainder iron. The highexpanding side of the bimetal has a first coefficient of thermalexpansion of from 0 F to 150 F of at least 7.0 X per "F and a secondcoefficient of thermal expansion lower than the first over a temperaturerange from 150 F to 600 F l 'he low expanding side of the bimetal has afirst coefficient of expansion ranging from about 1.4 10 per F to about6.0 X 10 per "F over a temperature range of 0 F to 150 F and a secondcoefficient of expansion of at least 7.0 X 10 per F over a temperaturerange. from about 400 F to 600 F. This particular bimetal reduces manyof the stresses found in prior art bimetals and results in improvedoverall operating conditions, especially in the higher temperatureranges. Depending on the configuration used for each of the bimetallicmembers, as well as the initial spacing between these members and thetemperature difference required to cause their engaging, otherbimetallic materials can be utilized successfully in this invention. Forpractical reasons however, bimetallic materials having operatingcharacteristics substantially similar to those described are preferred.

FIG. 5, taken along lines 5-5 in FIG. 4, more clearly illustratesanother feature of sensor 13. Herein, a pair of angled contactingportions 57 and 59, preferably comprised of the better electricallyconducting noble metals such as gold or silver, are shown attached tothe ends of first and second bimetallic members 47 and 49 respectively.Angled as shown, contacting portions 57 and 59 provide a means wherebymembers 47 and 49 more readily engage and assure that electricalconduction between the members is enhanced. Other configurations ofcontacting portions 57 and 59 are possible, however, with thoseillustrated not meant to so restrict the proposed invention.

In addition to providing means whereby a visual indication is given whenthe fluid within container 23 is below a certain level, apparatus 10 canbe modified to perform other functions by relatively simple alterationsto circuit 11, such as energizing other circuits, or to operate othermechanisms such as audible. signals, valving arrangements and the like.For example, circuit 11 can be modified to include a means for actuatinga valving arrangement in a boiler to thereby either shut down a burneror to open a valve and permit more fluid to enter the container,depending on the pre-established fluid level setting.

Besides this particular application, apparatus 10 may be also used inother situations in which the fluids to be detected vary in temperaturethroughout their cycles of operation. Primary examples of suchsituations are found in the several containers of fluids utilized in theoperation of automobiles. Sensor device 13 could easily be inserted intoan automobile s brake fluid housing, radiator side wall, or the varioushousings for the transmission fluid, engine oil, power steering fluid,differential fluid, or even the windshield washer fluid, with theautomobiles electrical circuitry readily able to substitute for circuit11.

Thus, there has been provided an apparatus for detecting the presence orabsence of fluid at a predetermined level within a container. Uniquefeatures of this apparatus, which include temperature compensation meansfor varying fluid temperatures and sequentially actuated currentindicating means, have also been provided.

While there has been shown and described what are I at presentconsidered the preferred embodiments of the invention, it will beobvious to those skilled in the art that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

What is claimed is:

1. A sensor device comprising:

a housing member defining a chamber;

a tip member adapted for being exposed to a fluid,

said tip member secured to said housing and forming a closure for saidchamber and to prevent fluid entrance into said chamber;

a heat conductive member within said chamber having a greater heattransfer capability than said tip member;

heater means within said chamber having first and second spaced apartportions, said first portion in heat conductive relationship to saidheat conductive member within said chamber, said second portion in heatconductive relationship to said tip member; and

first and second heat responsive means having substantially similarcharacteristics of thermal deflection, said first and second heatresponsive means in heat conductive relationship to said first andsecond spaced apart portions, respectively, and each having a free endadapted for mutual engagement only when the temperature differencebetween said first and second spaced apart portions of said heater meansexceeds a predetermined level, said temperature difference resultingfrom a difference in rates of heat transfer of said heat conductivemember and said tip member.

2. A sensor device according to claim 1 wherein said heater means withinsaid chamber is an electrical resistive element.

3. A sensor device according to claim 1 wherein said heat conductivemember within said chamber is electrically conductive.

4. A sensor device according to claim 1 wherein said tip member iselectrically conductive.

5. A sensor device according to claim 1 wherein said heat responsivemeans are bimetallic members.

6. The sensor device according to claim 5 wherein each of said first andsecond bimetallic members has a contacting portion of noble metalaffixed thereto.

7. A sensor device according to claim 5 wherein said bimetallic membershave a high expanding side of an alloy of nickel-chromium-iron and a lowexpanding side of an alloy of nickel-iron and wherein the differentialin coefficients of thermal expansion between said high and low expandingsides is appreciably greater at from about 0F to about 150F than fromabout 400F to about 600F.

8. A sensing apparatus comprising a sensor device comprising:

a housing member defining a chamber;

a tip member adapted for being exposed to a fluid,

said tip member secured to said housing and forming a closure for saidchamber and to prevent fluid entrance into said chamber;

a heat conductive member within said chamber having a greater heattransfer capability than said tip member;

heater means within said chamber having first and second spaced apartportions, said first portion in heat conductive relationship to saidheat conductive member within said chamber, said second portion in heatconductive relationship to said tip member;

first and second heat responsive means having substantially similarcharacteristics of thermal deflection, said first and second heatresponsive means in heat conductive relationship to said first andsecond spaced apart portions, respectively, and each having a free endadapted for mutual engagement only when the temperature differencebetween said first and second spaced apart portions of said heater meansexceeds a predetermined level, said temperature difference resultingfrom a difference in rates of heat transfer of said heat conductivemember and said tip member; and

means for sensing when said heat responsive means are engaged.

9. A sensing apparatus according to claim wherein said means for sensingwhen said heat responsive means are engaged is an electrical circuitcomprising a source of electrical potential operatively connected tosaid heat responsive means for opening and closing said circuit and acurrent indicating means adapted for indicating electrical current insaid circuit when said heat responsive means are engaged.

10. A sensing apparatus according to claim 9 wherein the said sensordevice said heat responsive means are bimetallic members.

11. A sensing apparatus according to claim 9 wherein in said sensordevice, said heater means within said chamber is an electrical resistiveelement and is operatively connected to said source of electricalpotential.

5 12. A sensing apparatus according to claim 11 wherein in said sensordevice said heat conductive member is electrically conductive and isoperatively connected to said electrical potential and to saidelectrical resistive element.

13. A sensing apparatus according to claim 12 wherein in said sensordevice said tip member is electrically conductive and is operativelyconnected to said source of electrical potential and said electricalresistive element.

14. In combination, a container having walls adapted for containing afluid therein, a sensor device at least partially positioned within awall of said container, said device comprising:

a housing member defining a chamber;

a tip member adapted for being exposed to a fluid, said tip membersecured to said housing and form ing a closure for said chamber and toprevent fluid entrance into said chamber;

a heat conductive member within said chamber having a greater heattransfer capability than said tip 25 member;

heater means within said chamber having first and second spaced apartportions, said first portion in. heat conductive relationship to saidheat conduc tive member within said chamber, said second portion in heatconductive relationship to said tip member;

first and second heat responsive means having substantially similarcharacteristics of thermal deflection, said first and second heatresponsive means in heat conductive relationship co said first andsecond spaced apart portions respectively, and each having a free endadapted for mutual engagement only when the temperature differencebetween said first and second spaced apart portions of said heater meansexceeds a predetermined level, said temperature difference resultingfrom a difference in rates of heat transfer of said heat conductivemember and said tip member; and

means for sensing when said heat responsive means are engaged.

15. A combination according to claim 14 wherein said means for sensingwhen said heat responsive means are engaged is an electrical circuitcomprising a source of electrical potential operatively connected tosaid heat responsive means for opening and closing said circuit and acurrent indicating means adapted for indicating electrical current insaid circuit when said heat responsive means are engaged.

16. A combination according to claim 15 wherein in said sensor devicesaid heat responsive means members are bimetallic members.

17. A combination according to claim 15 wherein in said sensor devicesaid heater means within said chamber is an electrical resistive elementand is operatively connected to said source of electrical potential.

18. A combination according to claim 17 wherein in said sensor devicesaid heat conductive member is electrically conductive and isoperatively connected to said electrical potential and to saidelectrical resistive element.

19. A combination according to claim 18 wherein in said sensor devicesaid tip member is electrically conductive and is operatively connectedto said source of electrical potential and said electrical resistiveelement. t III II t I

1. A sensor device comprising: a housing member defining a chamber; atip member adapted for being exposed to a fluid, said tip member securedto said housing and forming a closure for said chamber and to preventfluid entrance into said chamber; a heat conductive member within saidchamber having a greater heat transfer capability than said tip member;heater means within said chamber having first and second spaced apartportions, said first portion in heat conductive relationship to saidheat conductive member within said chamber, said second portion in heatconductive relationship to said tip member; and first and second heatresponsive means having substantially similar characteristics of thermaldeflection, said first and second heat responsive means in heatconductive relationship to said first and second spaced apart portions,respectively, and each having a free end adapted for mutual engagementonly when the temperature difference between said first and secondspaced apart portions of said heater means exceeds a predeterminedlevel, said temperature difference resulting from a difference in ratesof heat transfer of said heat conductive member and said tip member. 2.A sensor device according to claim 1 wherein said heater means withinsaid chamber is an electrical resistive element.
 3. A sensor deviceaccording to claim 1 wherein said heat conductive member within saidchamber is electrically conductive.
 4. A sensor device according toclaim 1 wherein said tip member is electrically conductive.
 5. A sensordevice according to claim 1 wherein said heat responsive means arebimetallic members.
 6. The sensor device according to claim 5 whereineach of said first and second bimetallic members has a contactingportion of noble metal affixed thereto.
 7. A sensor device according toclaim 5 wherein said bimetallic members have a high expanding side of analloy of nickel-chromium-iron and a low expanding side of an alloy ofnickel-iron and wherein the differential in coefficients of thermalexpansion between said high and low expanding sides is appreciablygreater at from about 0*F to about 150*F than from about 400*F to about600*F.
 8. A sensing apparatus comprising a sensor device comprising: ahousing member defining a chamber; a tip member adapted for beingexposed to a fluid, said tip member secured to said housing and forminga closure for said chamber and to prevent fluid entrance into saidchamber; a heat conductive member within said chamber having a greaterheat transfer capability than said tip member; heater means within saidchamber having first and second spaced apart portions, said firstportion in heat conductive relationship to said heat conductive memberwithin said chamber, said second portion in heat conductive relationshipto said tip member; first and second heat responsive means havingsubstantially similar characteristics of thermal deflection, said firstand second Heat responsive means in heat conductive relationship to saidfirst and second spaced apart portions, respectively, and each having afree end adapted for mutual engagement only when the temperaturedifference between said first and second spaced apart portions of saidheater means exceeds a predetermined level, said temperature differenceresulting from a difference in rates of heat transfer of said heatconductive member and said tip member; and means for sensing when saidheat responsive means are engaged.
 9. A sensing apparatus according toclaim 10 wherein said means for sensing when said heat responsive meansare engaged is an electrical circuit comprising a source of electricalpotential operatively connected to said heat responsive means foropening and closing said circuit and a current indicating means adaptedfor indicating electrical current in said circuit when said heatresponsive means are engaged.
 10. A sensing apparatus according to claim9 wherein the said sensor device said heat responsive means arebimetallic members.
 11. A sensing apparatus according to claim 9 whereinin said sensor device, said heater means within said chamber is anelectrical resistive element and is operatively connected to said sourceof electrical potential.
 12. A sensing apparatus according to claim 11wherein in said sensor device said heat conductive member iselectrically conductive and is operatively connected to said electricalpotential and to said electrical resistive element.
 13. A sensingapparatus according to claim 12 wherein in said sensor device said tipmember is electrically conductive and is operatively connected to saidsource of electrical potential and said electrical resistive element.14. In combination, a container having walls adapted for containing afluid therein, a sensor device at least partially positioned within awall of said container, said device comprising: a housing memberdefining a chamber; a tip member adapted for being exposed to a fluid,said tip member secured to said housing and forming a closure for saidchamber and to prevent fluid entrance into said chamber; a heatconductive member within said chamber having a greater heat transfercapability than said tip member; heater means within said chamber havingfirst and second spaced apart portions, said first portion in heatconductive relationship to said heat conductive member within saidchamber, said second portion in heat conductive relationship to said tipmember; first and second heat responsive means having substantiallysimilar characteristics of thermal deflection, said first and secondheat responsive means in heat conductive relationship co said first andsecond spaced apart portions respectively, and each having a free endadapted for mutual engagement only when the temperature differencebetween said first and second spaced apart portions of said heater meansexceeds a predetermined level, said temperature difference resultingfrom a difference in rates of heat transfer of said heat conductivemember and said tip member; and means for sensing when said heatresponsive means are engaged.
 15. A combination according to claim 14wherein said means for sensing when said heat responsive means areengaged is an electrical circuit comprising a source of electricalpotential operatively connected to said heat responsive means foropening and closing said circuit and a current indicating means adaptedfor indicating electrical current in said circuit when said heatresponsive means are engaged.
 16. A combination according to claim 15wherein in said sensor device said heat responsive means members arebimetallic members.
 17. A combination according to claim 15 wherein insaid sensor device said heater means within said chamber is anelectrical resistive element and is operatively connected to said sourceof electrical potential.
 18. A combination according to claim 17 whereinin said sensor device said heat conductivE member is electricallyconductive and is operatively connected to said electrical potential andto said electrical resistive element.
 19. A combination according toclaim 18 wherein in said sensor device said tip member is electricallyconductive and is operatively connected to said source of electricalpotential and said electrical resistive element.